Mononuclear cells when stimulated with concanavalin A develop different biological activities, including suppressor activity. The objective of this work is to find out whether the inhibitory activity is mediated through soluble factors, and to characterize these factor's biological and physiochemical properties. We have found that the factors responsible for suppression are nondialyzable, heat stable, resistant to pH2 treatment and inhibits proliferation of a variety of cells including human lymphocytes, retinoblastoma cells and stromal keratocytes.